The present invention relates generally to information storage systems and, more particularly, to detection of data retrieved from storage in such systems
Digital data magnetic recording, or storage and retrieval, systems store digital data by recording same in a moving magnetic media layer using a storage, or xe2x80x9cwritexe2x80x9d, electrical current-to-magnetic field transducer, or xe2x80x9cheadxe2x80x9d, positioned immediately adjacent thereto. The data is stored or written to the magnetic media by switching the direction of flow in an otherwise substantially constant magnitude write current that is established in coil windings in the write transducer in accordance with the data. Each write current direction transition results in a reversal of the magnetization direction, in that portion of the magnetic media just then passing by the transducer during this directional switching of the current flow, with respect to the magnetization direction in that media induced by the previous in the opposite direction. In one recording scheme, often termed non-return-to-zero inverted (NRZI), each magnetization direction reversal occurring over a short portion of the magnetic media moving past the transducer represents a binary number system digit xe2x80x9c1xe2x80x9d, and the lack of any such reversals in that portion represents a binary digit xe2x80x9c0xe2x80x9d.
Recovery of such recorded digital data is accomplished through positioning a retrieval, or xe2x80x9creadxe2x80x9d magnetic field-to-voltage transducer, (which may be the same as the storage transducer if both of these transducers rely on inductive coupling between the media fields and the transducer) or xe2x80x9cheadxe2x80x9d, is positioned to have the magnetic media, containing previously stored data, pass thereby. Such passing by of the media adjacent to the transducer permits the flux accompanying the magnetization reversal regions in that media either to induce a corresponding voltage pulse in forming an analog output read signal for that retrieval transducer or, alternatively, change a transducer circuit parameter to thereby provide such an output signal voltage pulse. In the coding scheme described above, each such voltage pulse in the read transducer output signal due to the reversal of magnetization directions between adjacent media portions is taken to represent a binary digit xe2x80x9c1xe2x80x9d, and the absence of such a pulse in corresponding media portions is taken to represent a binary digit xe2x80x9c0xe2x80x9d.
Digital data magnetic recording systems have used peak detection methods for the detection of such voltage pulses in the retrieved analog signal as the basis for digitizing this signal. Such methods are based on determining which peaks in that signal exceed a selected threshold to determine that a binary digit xe2x80x9c1xe2x80x9d related pulse occurred in the retrieved signal, and also use the times between those voltage pulses to reconstruct the timing information used in the preceding recording operation in which the data were stored in the magnetic media as described above. The analog retrieved signal is provided to a phase-locked loop forming a controlled oscillator, or a phase-locked oscillator or synchronizer, which produces an output timing signal, or xe2x80x9cclockxe2x80x9d signal, from the positions of the detected peaks in this analog retrieved signals Absolute time is not used in operating the data retrieval system portion since the speed of the magnetic media varies over time during both the storage operation and the retrieval operation to result in nonuniform time intervals, or nonuniform multiples thereof, occurring between the voltage pulses in the analog retrieved signal.
There is always a desire in magnetic recording systems to devote less of the magnetic media along a track therein to the storage of a bit to thereby permit increasing the density of the bits stored. The use of peak detection places a limit on the density of bits along a track because increasing that density beyond some point will lead to too much intersymbol interference which in turn leads to errors in the recovery of data using such peak detection methods. Because of this limit, recent increases in bit density along a track in a magnetic media have come with the acceptance of a controlled, or known, amount of intersymbol interference which, since known, allows detection of the pulses involved despite this interference. The read transducer analog output signal generated from the binary bits or symbols stored in the magnetic media is sampled with the resulting samples being converted to digital data, and the samples are taken at a rate which leads to more than one sample per pulse rather than the single sample per pulse which would be sufficient for peak detection if sampling was used therewith. Since each individual sample reflects only part of the pulse response, this process used in a system results in referring to such a system as a partial response system.
A digital data magnetic recording system typically comprises a bandpass data retrieval channel in that it is unable to transmit very low frequencies, and in that it has an upper frequency beyond which its transmission is also quite limited, and is often termed a Lorentzian channel in view of the resulting response to an isolated pulse. Thus, the channel, including any equalizer therein, should exhibit in passing a stream of data pulses therethrough a spectral characteristic having spectral nulls at zero frequency and at a frequency equal to half the symbol rate or pulse rate. Channels having an impulse response characteristic of the form (1xe2x88x92D) (1+D)k for such a stream have been found to provide such nulls but at the cost of accepting a substantial amount of intersymbol interference leading to the description of the channel as a xe2x80x9cpartial response system.xe2x80x9d Here, D is the unit delay operator, or D=ejxcfx89T where T is the bit period and kxe2x89xa7I for the characteristic described. Such channel response characteristics have integer coefficients resulting in the sampled channel output symbols also having only integer values.
Although there are a number of possible alternative partial response system arrangements, there is substantial value in choosing a channel characteristic of the above form that has the smallest value for n that is possible for the data to be transmitted in the channel. Increasing the value of n leads to increased intersymbol interference resulting in more output symbol values thereby reducing the signal-to noise ratio and increasing the necessary detector complexity and performance requirements. The simplest partial response system with the desired characteristics results from k being set equal to one to yield a channel characteristic of (1xe2x88x92D) (1+D)=1xe2x88x92D2 that is known as a class 4 partial response system, and has been typically used previously in magnetic digital data recording Systems. Such a response is obtained by providing an overall channel and filter response equal to that of the sum of two opposite polarity Nyquist channel impulse responses separated in time by two sample intervals. Such an arrangement will lead to a filter analog output signal from which ideally can be obtained three alternative possible output symbol sample values of xe2x88x921, 0 and 1 for an input signal based on binary recorded data if sampled at appropriate instants.
A range of lineal bit representation densities along a track in the magnetic media, leading to a range of data pulse rates for retrieved data, can be accommodated by providing an equalizer in the channel suited to the density chosen. Such an equalizer operates by changing the effective channel characteristic as to keep the output symbol samples at the proper integer values. However, as the lineal density is increased along the tracks in the magnetic media a point is reached where the noise enhancement provided by the equalizer becomes unacceptable. This situation, along with other considerations, requires going to a greater value for n for further density increases to allow a reduction in the transmittals at frequency values.
The next higher values for k are 2 and 3, and these values are appropriate at significantly higher user densities, i.e. when pw50/Tuser is at 2.4 or above where pw50 is the pulse width in time for an isolated magnetic pulse between its half peak amplitude values, and Tuser is the time per user bit which is equal to the bit time T divided by R, the code rate for any modulation code being used. When k 2, the channel is referred to as an extended class 4 partial response system which is usually written as E2PR4. When k=3, the channel is usually written as E2PR4. A read transducer analog output signal provided through any kind of a data retrieval channel is subject to containing errors therein as a result due to noise, timing errors, gain errors, channel asymmetries and the like encountered in the course of retrieval. As opposed to attempting to determine individually the value represented by every pulse in the read signal as in peak detection, maximum likelihood detection of sequences of such samples is used instead involving estimating which of several possible transmitted symbol sequences caused the received sample sequence. This determination is typically based on finding the minimum mean squared error between these received samples and each of the possible symbol sequences that may have generated these samples, and then choosing that symbol sequence giving the smallest such error.
The detection process over time for a channel receiving data can be illustrated by a multiple state trellis diagram showing the possible sequence of detection system states resulting from transitions therebetween that can evolve over time. States are shown in the diagram after each corresponding sample interval and the transitions between these states which can occur on obtaining the next data symbol after the subsequent sample interval are shown leading to the next states. Each transition is represented by an arrow, or branch, usually with a pair of values in parentheses adjacent thereto showing a possible next expected data sample after being in the preceding state, and the corresponding data output symbol value on the right. Such a trellis diagram is shown assuming the detection system was initially in some state and is depicted for a sequence of samples of arbitrary length, often for the length of a code word or block, the diagram having each sample interval marked below the corresponding states the system could be in at that instant. A sequence of contiguous branches from the beginning to the end is a path through the diagram corresponding to a possible input sequence of data symbol values.
There will be some correct state sequence, or path, through the trellis diagram representing the detection process for the data symbol sequence recovered from storage in the magnetic media, and there will be a state sequence, or path, through the trellis diagram reflecting the selections of the detection system using the Viterbi algorithm. Due to conditions in the retrieval channel, errors can occur from time to time as indicated above so that these two paths in the trellis diagram will correspondingly separate and thereafter merge together again. Each of such occurrences is termed an error event of a length depending on how many states occur between such a separation and the subsequent remerging of the paths.
Such a trellis diagram representing a detector is usually based on coded sequences of data which may represent multiple codings of that data for different purposes. Modulation coding is usually used because such use can compensate for shortcomings of the channel such as nonlinearities present therein or to aid the operation of the detection system such as aiding the recovery of a clocking signal from the retrieved data to be used in coordinating the retrieval system. In addition, such coding can provide increased distances between code words to aid the data retrieval process typically by eliminating the most common error events. Analysis of an equalized channel having initially retrieved signals introduced into a Lorentzian channel subject to white, Gaussian noise all passed through a low pass filter to an equalizer and a sampler shows the smallest distance, and so most common, error events is the failure to detect data sequences that are stored in the magnetic media as three or more consecutive magnetization direction transitions, i.e. three or more stored consecutive digital binary values of xe2x80x9c1xe2x80x9d.
Codes have been devised that eliminate such error events both for E2PR4 and E2PR4 equalized channels. Use of such codes in a magnetic data storage and retrieval system has often been found to also require use of a Viterbi algorithm based periodically time-varying characteristic data detector for which the characterizing trellis will have selected state transition edges omitted or added as a function of time to reduce the possibilities of errors occurring in the detection process. At higher data retrieval rates possible in an E2PR4 equalized channel, there are advantages in using a radix-4 detector. In this circumstance in which consecutive pairs of channel samples are received by the detector with the members of each pair processed concurrently by that detector. Such a radix-4 detector, (providing detection processing for consecutive pairs of samples of the data retrieval channel equalized analog signal) has each branch in the detector trellis representation being representative of two such samples (noiseless) and the state transition result for the corresponding two channel input bit values. The use of a Viterbi algorithm based periodically time-varying characteristic radix-4 detector leads to not only needing to have a time-varying detector operated with proper timing with respect to the code blocks but also with proper phasing with respect to each sample pair member. Hence, a synchronizer is desirable for operation of the detector which enables the detector to start in a known state at a selected time with respect to the timing cycle used in its periodic variation and to be in phase with the data samples.
A method and apparatus for synchronizing a Viterbi detector to data frames corresponding to code words read from a medium is provided. The method includes providing a timing pattern, providing a synchronization pattern, synchronizing the detector based upon the timing pattern; and synchronizing the detector to the data frame based upon the synchronization pattern.